Power over Ethernet (PoE), which is outlined in IEE Std 802.3™-2005 clause 33 (the PoE standard), refers to a technique for delivering power and data to an electronic device via a network cable, such as a twisted pair Ethernet cable. Typically, a PoE system includes power sourcing equipment (PSE) device to provide a power supply via Ethernet cabling to one or more electronic devices, which may be referred to as powered devices. PoE reduces the need for a separate power source to deliver power to attached powered devices, which may include Voice over Internet protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and other devices.
Generally, since PSE devices include interfaces for connecting to cables, PSE devices may be exposed to transient conditions, such as electrostatic discharge events, high voltage surges, or other conditions that exceed a power rating of associated circuitry. The PoE standard specifies that a PSE device should be capable of withstanding high voltage (HV) transient conditions for a period of time, such as 1000 volts having 300 μs full rise time and a 50 ns half rise time.
In general, a PSE device applies power from an external power supply (approximately plus or minus 44 volts to 57 volts) to an Ethernet cable through a switch device. The switch device may be a transistor to selectively couple the external power supply to the Ethernet line or channel. Since telephone lines typically operate at a negative voltage (such as −48 volts), the PSE device may apply a negative voltage to the Ethernet line.
In some PSE devices, the switch device may be integrated. Under normal operating conditions, the switch device may sustain currents as high as 400 mA with low voltage (e.g. 44 volts to 57 volts), so the power dissipation in the switch device is reasonably low. When a transient or power surge event occurs, an external protector may be activated to provide some protection, but the switch device may not turn off because the switch device may not detect the transient or power surge event. In such an instance, the switch device may experience both a high current (e.g. 400 mA) and a high voltage (e.g. 1000 volts), and the resulting power dissipation through the switch device may cause a failure. Hence, there is a need for an improved voltage protection circuit in power supply equipment.